add two test cases

#1. test_unlock_unheld_lock
    If the lock was not held, pg_advisory_unlock will return false.
#2. test_consecutive_acquire
    If a session already holds a given advisory lock,
    additional requests by it will always succeed,
    even if other sessions are awaiting the lock.
Merge pull request #1 from quiver/more-testcases

add two more test cases